Truck trailers used to transport perishable and frozen goods include a refrigerated trailer pulled behind a track cab unit. The refrigerated trailer, which houses the perishable or frozen cargo, requires a refrigeration unit for maintaining a desired temperature environment within the interior volume of the container. The refrigeration unit must have sufficient refrigeration capacity to maintain the product stored within the trailer at the desired temperature over a wide range of ambient air temperatures and load conditions. Refrigerated trailers of this type are used to transport a wide variety of products, ranging for example from freshly picked produce to deep frozen seafood. Product may be loaded into the trailer unit directly from the field, such as freshly picked fruits and vegetables, or from a warehouse.
One type of transport refrigeration system used in connection with truck trailers includes an electrically powered refrigeration unit operatively associated with the trailer. The refrigeration unit includes a hermetic, motor driven, reciprocating refrigerant compressor, a condenser heat exchanger unit, an expansion device, commonly an electronic or a thermostatic expansion valve (TXV), and an evaporator heat exchanger unit connected via appropriate refrigerant lines in a closed refrigerant circuit, and an engine driven electric generator. The refrigeration unit, the generator and the engine that drives the generator are contained in a framework that is attached to the front wall of the trailer behind the truck cab such that the air or gas/air mixture or other gas within the interior volume of the trailer may be circulated over the evaporator coil of the evaporator heat exchanger unit by means of an evaporator fan associated with the evaporator coil which is disposed within the interior of the trailer, typically mounted in an opening in the front wall to which the refrigeration unit is attached. The electric generator driven by a diesel powered engine and adapted to produce AC current at a selected voltage and frequency to power a compressor drive motor driving the reciprocating refrigeration compressor, at least one condenser fan motor, at least one evaporator fan motor, and all other electric powered devices associated with the refrigeration unit. For example, U.S. Pat. No. 6,223,546, assigned to Carrier Corporation, discloses a transport refrigeration unit configured to be mounted on the front wall of a refrigerated transport trailer.
It is desirable that the truck/trailer refrigeration units operate efficiently over a wide range of refrigeration capacity demands. The refrigeration unit must be capable of providing sufficient refrigerant capacity during “pulldown” to, within a limited time, reduce the temperature within the cargo box of the trailer when newly loaded perishable product, which may be field-loaded at ambient temperature, down to the desired storage temperature. The refrigerant unit must also be capable of providing sufficient refrigerant capacity to maintain a low box temperature for frozen product (0° C. (32° F.)) or for deep frozen product (−18° C. (about 0° C.)) even under high ambient temperature conditions. The refrigeration unit should also be capable of efficient operation at very low refrigeration capacity, for example, when transporting a product that is to be stored at or near ambient temperature.
Although electrically powered trailer refrigeration units have exhibited improved reliability, due in part to the reciprocating compressor being motor driven as opposed to belt driven, the electrical losses due to the all electric architecture of current electrically powered trailer refrigeration units present challenges when designing a system to meet refrigeration capacity targets. The more electric power available, the more refrigeration capacity potential an all electric refrigeration unit has. To compensate for electrical losses, engine, generator, compressor, fan and coil sizing are typically increased in order to meet the system refrigeration capacity requirements, which has a direct effect on system cost, fuel efficiency, emissions, and available design envelope. In some locations, refrigerated trailer designs are standard and have specific requirements regarding the size that a refrigeration unit can be. This results in significant design space limitations.